P-type Transport Research Articles

Molecular Weight and Subunit Composition of the Sensitive to GABA-Ergic Ligands Cl−/HCO 3 − -Stimulated Mg2+-ATPase from Plasma Membrane of Rat Brain

The molecular weight and subunit composition of Cl-,HCO3(-)- and picrotoxin-stimulated Mg2+-ATPase from rat brain plasma membrane solubilized in sodium deoxycholate were studied by gel filtration chromatography. The enzyme activity eluted from a Sephacryl S-300 column in a single peak associated with a protein of molecular weight approximately 300 kD and a Stokes radius of 5.4 nm. The enzyme-enriched fraction, concentrated and denatured by SDS, migrated through a Sephacryl S-200 column as three peaks with molecular weights of approximately 57, 53, and 45 kD. SDS-PAGE also showed three major protein bands with molecular weights of about 57, 53, and 48 kD. The molecular weight and subunit composition of the Cl- and HCO3(-)-stimulated Mg2+-ATPase from neuronal membrane of rat brain are similar with the molecular properties of GABA(A)-benzodiazepine receptor complex from mammalian brain but are different from those of P-type transport ATPases.

Redox behavior of acceptor-doped La(Al,Fe)O 3− δ

In order to study the behavior of perovskite-type phases containing similar fractions of transition metal cations and ions with stable oxidation state in the B sublattice, the defect formation processes in mixed-conducting La 0.90Sr 0.10Al 0.45Fe 0.40Mg 0.15O 3− δ were analyzed using the measurements of total conductivity in the oxygen partial pressure range from 10 −20 to 0.5 atm at 1073–1223 K. The results, in combination with Mössbauer spectroscopy data and ion transference numbers determined by the faradaic efficiency technique in air, show that increasing p(O 2) leads to decreasing oxygen ionic conductivity, whilst no essential delocalization of the electronic charge carriers is observed. The variations of partial ionic and p- and n-type electronic conductivities can be adequately described by equilibrium processes of oxygen intercalation and iron disproportionation with the thermodynamic functions independent of defect concentrations. The hole mobility is also concentration-independent and has an activation energy of 37 ± 1 kJ/mol, suggesting that the p-type electronic transport occurs via a small-polaron mechanism within the whole p(O 2) range studied. The values of hole concentration derived from the conductivity data are in excellent agreement with the Mössbauer spectroscopy results.

Cerium–terbium mixed oxides as potential materials for anodes in solid oxide fuel cells

Highly homogeneous (Ce,Tb) oxides are prepared by a microemulsion technique, and their structural and electronic state after high temperature calcination is examined with X-ray diffraction, high resolution transmission electron microscopy, X-ray photoelectron and absorption (XANES) spectroscopies and impedance spectroscopy measurements. Addition of Tb stabilizes significantly (in comparison to pure ceria) specific surface area and small particles sizes during high temperature calcination (up to 1100 °C); phase decomposition at these high temperatures, similar to that occurring when stabilization of ceria is carried out with Zr, does not occur, and the mixed oxide remains homogeneous throughout. Tb addition to ceria may thus be beneficial when used as a component of SOFC anodes. TEM data indicate reshaping of oxide particles and provide evidence of crystal superstructures after high temperature treatments, while XPS and XANES reveal an increase in the Tb 4+/Tb 3+ ratio (for a given pretreatment) with the Tb/Ce ratio; Ce seems to be less reducible to Ce 3+ in the presence of Tb. Total electrical conductivity of CT samples under H 2 is mediated by electron transport (involving probably only Ce) and is lower than in gadolinia-doped ceria (GCO); in air conductivity is higher than for GCO, particularly at low temperatures, and it is probable that a p-type transport mechanism predominates in this case.

Characterization of the P5 subfamily of P-type transport ATPases in mice

In mammals, the most poorly understood P-type ATPases are those of the P5 subfamily. To begin characterization of the mammalian P5-ATPases, BLAST searches of DNA sequence databases were performed. Five genes were identified in the mouse, human, dog, and rat genomes, and the coding sequences of the mouse genes, termed Atp13a1–Atp13a5, were determined. The intron/exon organization of Atp13a1 differs entirely from those of Atp13a2-5, which are closely related. Amino acid sequence comparisons between the five mouse and two yeast P5-ATPases suggest that Atp13a1 is orthologous to the yeast Cod1 gene and that Atp13a2-5 are orthologous to yeast Yor291w. Northern blot analysis showed that Atp13a1, Atp13a2, and Atp13a3 mRNAs were expressed in all mouse tissues, whereas Atp13a4 and Atp13a5 mRNAs were restricted to brain and stomach. While the substrate specificity of these transporters is unknown, their importance is underscored by the presence of homologs in fish, insects, worms, and other eukaryotes.

Alkylidene Fluorene Liquid Crystalline Semiconducting Polymers for Organic Field Effect Transistor Devices

Organic electronic devices comprising arrays of organic field effect transistors (OFETs) are expected to create a range of novel applications for which the ability to be fabricated in large areas, on flexible substrates, with nonconventional shapes, and at low cost are key enabling factors. To improve the electrical performance of such devices, new solution processable organic semiconductors are required with high charge carrier mobilities and environmental stability. This work describes the molecular design of a p-type charge transport liquid crystalline polymer, in an attempt to control the factors responsible for both mobility and stability. Molecules were designed that were able to exhibit closely packed, π stacked morphologies, which can result in efficient intermolecular charge hopping and hence high mobility. Molecular manipulation of the conjugated π electron system was required to optimize the HOMO energy level, to both resist oxidation and be able to readily accept holes from a source electrode.

Operational stability of electrophosphorescent devices containing p and n doped transport layers

The operational stability of low-operating voltage p-i-n electrophosphorescent devices containing fac-tris(2-phenylpyridine) iridium as the emissive dopant is investigated. In these devices, Li-doped 4,7-diphenyl-1,10-phenanthroline (BPhen) served as an n-type electron transport layer, or as an undoped hole blocking layer (HBL), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane doped 4,4′,4″-tris(3-methylphenylphenylamino) triphenylamine served as a p-type hole transport layer. The glass transition temperature of BPhen can be increased by the addition of aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq), resulting in improved morphological stability, thereby reducing device degradation. When thermally stable BAlq was used as a HBL in both p-i-n and undoped devices, the extrapolated operational lifetime (normalized to an initial luminance of 100 cd/m2) of the p-i-n and undoped devices are 18 000 and 60 000 h, respectively, indicating that the presence of p and n dopants can accelerate device degradation.

Mechanically-Activated Synthesis and Mixed Conductivity of TbMO4-δ(M = Zr, Hf) Ceramics

Terbium hafnate and zirconate ceramics with submicron grain sizes were prepared via mechanically-activated synthesis. X-ray and electron diffraction and infrared (IR) absorption spectroscopy showed that TbZrO4−δ has a disordered fluorite-type structure, while TbHfO4−δ is partially ordered, containing pyrochlore microdomains. The oxygen ion transference numbers determined by the modified e.m.f. technique under oxygen/air gradient, vary in the range 0.08–0.26 at 873–1123 K, increasing with temperature. The activation energies for ionic and p-type electronic transport are 82–83 and 29–40 kJ/mol, respectively. The ionic conduction becomes dominant in reducing atmospheres, but tends to decrease at low p(O2). Oxygen partial pressure dependencies of Seebeck coefficient can be described by a model common for oxide phases with mixed ionic and electron-hole conductivity. Due to partial cation ordering, terbium hafnate exhibits lower ionic and hole transport as compared to TbZrO4−δ. The average thermal expansion coefficients of TbMO4−δ (M = Zr, Hf) ceramics in air, calculated from dilatometric data, are (11.5–12.4) × 10−6 K−1 at 600–1200 K and (18.4–20.3) × 10−6 K−1 at 1200–1420 K.

Cu-Ce0.8Gd0.2O2−δ materials as SOFC electrolyte and anode

Co-sintering of Cu-CGO cermet anodes on CGO (Ce0.8Gd0.2O2−δ) electrolyte was conducted at low temperature (1000 °C) by introducing small amounts (2 mol.%) of CuO sintering aid to the electrolytic CGO. The Cu-CGO anodes with Cu contents from 20–50 vol.% were prepared by combustion synthesis followed by sintering and reduction. Symmetrical anode/electrolyte/anode assemblies of Cu-GCO/CGO/Cu-CGO were fabricated by manually depositing the anode combustion powder on a green substrate of the 2 mol% CuO-containing CGO, followed by co-pressing and co-sintering of the assembly at 1000 °C. The low-temperature sintered CGO is submicron with 95–99% relative density. CuO addition has no significant effect on either the total or ionic conductivity of the electrolyte, but p-type conduction in the temperature range, 900–1200 °C, is 25 times higher than that of undoped CGO. Oxygen-ion transference numbers of the Cu-containing CGO lie in the range 0.89–0.99, as determined by the modified e.m.f. technique under an oxygen/air potential gradient. The activation energy for ionic conduction, 83 kJmol−1, is significantly lower than that for p-type electronic transport, 140 kJmol−1.

Oxygen ionic and electronic transport in Gd2?xCaxTi2O7 ?? pyrochlores

Oxygen ion transference numbers for Gd2−xCaxTi2O7−δ (x=0.10–0.14) pyrochlore ceramics were determined at 973–1223 K by the modified e.m.f. and faradaic efficiency techniques, taking into account electrode polarization, and from the results on oxygen permeation. The ion transference numbers vary in the range 0.95–0.98 in air, increasing when the temperature or oxygen partial pressure decreases. The activation energies for the ionic and p-type electronic transport in air are 74–77 and 87–91 kJ/mol, respectively. The p-type conductivity and oxygen permeability of Gd2Ti2O7-based pyrochlores can be adequately described by relationships common for other solid electrolytes. At temperatures below 1273 K under a gradient of 10%H2+90%N2/air, average ion transference numbers for doped gadolinium titanate are not less than 0.97. Thermal expansion coefficients for Gd2−xCaxTi2O7−δ ceramics, calculated from dilatometric data in air, are in the range (10.4–10.6)×10−6 K−1 at 400–1300 K.

Self-assembled InP Quantum Dot -TiO2 Solid Grätzel Solar Cell

AbstractThe processing of Solid Grätzel Solar Cells (SGSCs) offers advantages of cost effective fabrication, possible flexible products, and high incident photon-to-electron conversion efficiencies (IPCE). In this paper, InP quantum dots (QDs) were synthesized and used as the photosensitizer to design new generation SGSCs. An organic p-type charge transport material (spiro-OMeTAD) was used as the hole-transport material (HTM). The InP QD has an average diameter of approximately 3nm. Preliminary research indicates that InP QD is a promising photosensitizer, which allows the sensitized SGSC TiO2 nanocrystalline cell to achieve over 400mV open circuit photovoltage, and a short-circuit current greater than 0.035mA/cm2 under the illumination of a solar simulator with an integrated light power of 58mW/cm2.

Quantum Dot Sensitization of Organic−Inorganic Hybrid Solar Cells

A high surface area pn-heterojunction between TiO2 and an organic p-type charge transport material (spiro-OMeTAD) was sensitized to visible light using lead sulfide (PbS) quantum dots. PbS quantum dots were formed in situ on a nanocrystalline TiO2 electrode using chemical bath deposition techniques.1 The organic hole conductor was applied from solution to form the sensitized heterojunction. The structure of the quantum dots was analyzed using HRTEM technique. Ultrafast laser photolysis experiments suggested the initial charge separation to proceed in the subpicosecond time range. Transient absorption laser spectroscopy revealed that interfacial charge recombination of the initially formed charge carriers is much faster than in comparable dye-sensitized systems.2,3 The sensitized heterojunction showed incident photon-to-electron conversion efficiencies (IPCE) of up to 45% and energy conversion efficiencies under simulated sunlight AM1.5 (10 mW/cm2) of 0.49%.

Structure and electronic conductivity of Bi 2− xLa xV 0.9Cu 0.1O 5.5− δ

The structure refinement of La-doped Bi 2V 0.90Cu 0.10O 5.5− δ (BICUVOX.10) showed that the maximum solid solubility of lanthanum cations in the bismuth sublattice corresponds to approximately 2–2.5% of the Bi site concentration. Doping with lanthanum leads to a decrease in both oxygen ionic and electronic conductivities of Bi 2− x La x V 0.9Cu 0.1O 5.5− δ ( x=0–0.20). The oxygen ion transference numbers determined by the modified e.m.f. method under the oxygen pressure gradient of 101/21 kPa at 820–970 K vary in the range 0.962–0.999, increasing with the decrease in temperature. Reduction of the oxygen partial pressure down to 10 kPa leads to a drastic increase of the n-type electronic conductivity and then to phase decomposition. In contrast to the instability of Bi 2V 0.90Cu 0.10O 5.5− δ at intermediate oxygen partial pressures, annealing at high oxygen pressures (up to 12 MPa) results in neither phase decomposition nor considerable increase of the p-type electronic transport.

P-Type Electronic Transport in Ce0.8Gd0.2O2 − δ: The Effect of Transition Metal Oxide Sintering Aids

Small (2 mol%) additions of cobalt, iron and copper oxides into Ce0.8Gd0.2O2 − δ considerably improve sinterability of ceria-gadolinia (CGO) solid electrolyte, making it possible to obtain ceramics with 95–99% density and sub-micron grain sizes at 1170–1370 K. The minor dopant additions have no essential effect on the total and ionic conductivity, whilst the p-type conduction in the transition metal-containing materials at 900–1200 K is 8–30 times higher than that in pure CGO. The oxygen ion transference numbers of the Co-, Fe- and Cu-doped ceramics, determined by the modified e.m.f. technique under oxygen/air gradient, are in the range 0.89–0.99. The electron-hole contribution to the total conductivity increases with temperature, as the activation energy for ionic conduction, 78 to 82 kJ/mol, is significantly lower than that for the p-type electronic transport (139–146 kJ/mol). The results show that CGO sintered with such additions can still be used as solid electrolytes for intermediate-temperature electrochemical applications, including solid oxide fuel cells (SOFCs) operating at 770–970 K, but that increasing operation temperature is undesirable due to performance loss.

Cell volume homeostasis: Ionic and nonionic mechanisms: The sodium pump in the emergence of animal cells

Plant cells and bacterial cells are surrounded by a massive polysaccharide wall, which constrains their high internal osmotic pressure (tens of atmospheres). Animal cells, in contrast, are in osmotic equilibrium with their environment, have no restraining surround, and can take on a variety of shapes and can change these from moment to moment. This osmotic balance is achieved, in the first place, by the action of the energy-consuming sodium pump, one of the P-type ATPase transport protein family, members of which are found also in bacteria. The pump's action brings about a transmembranal electrochemical gradient of sodium ions, harnessed in a range of transport systems which couple the dissipation of this gradient to establishing a gradient of the coupled substrate. These transport systems include many which are responsible for short-term regulation of the cell's volume in response to acute changes of their osmotic balance. Thus, the primary role of the sodium pump as a regulator of cell volume has been built upon to provide the basis for an enormous variety of physiological functions.

Role of Alternative Splicing in Generating Isoform Diversity Among Plasma Membrane Calcium Pumps

Calcium pumps of the plasma membrane (also known as plasma membrane Ca(2+)-ATPases or PMCAs) are responsible for the expulsion of Ca(2+) from the cytosol of all eukaryotic cells. Together with Na(+)/Ca(2+) exchangers, they are the major plasma membrane transport system responsible for the long-term regulation of the resting intracellular Ca(2+) concentration. Like the Ca(2+) pumps of the sarco/endoplasmic reticulum (SERCAs), which pump Ca(2+) from the cytosol into the endoplasmic reticulum, the PMCAs belong to the family of P-type primary ion transport ATPases characterized by the formation of an aspartyl phosphate intermediate during the reaction cycle. Mammalian PMCAs are encoded by four separate genes, and additional isoform variants are generated via alternative RNA splicing of the primary gene transcripts. The expression of different PMCA isoforms and splice variants is regulated in a developmental, tissue- and cell type-specific manner, suggesting that these pumps are functionally adapted to the physiological needs of particular cells and tissues. PMCAs 1 and 4 are found in virtually all tissues in the adult, whereas PMCAs 2 and 3 are primarily expressed in excitable cells of the nervous system and muscles. During mouse embryonic development, PMCA1 is ubiquitously detected from the earliest time points, and all isoforms show spatially overlapping but distinct expression patterns with dynamic temporal changes occurring during late fetal development. Alternative splicing affects two major locations in the plasma membrane Ca(2+) pump protein: the first intracellular loop and the COOH-terminal tail. These two regions correspond to major regulatory domains of the pumps. In the first cytosolic loop, the affected region is embedded between a putative G protein binding sequence and the site of phospholipid sensitivity, and in the COOH-terminal tail, splicing affects pump regulation by calmodulin, phosphorylation, and differential interaction with PDZ domain-containing anchoring and signaling proteins. Recent evidence demonstrating differential distribution, dynamic regulation of expression, and major functional differences between alternative splice variants suggests that these transporters play a more dynamic role than hitherto assumed in the spatial and temporal control of Ca(2+) signaling. The identification of mice carrying PMCA mutations that lead to diseases such as hearing loss and ataxia, as well as the corresponding phenotypes of genetically engineered PMCA "knockout" mice further support the concept of specific, nonredundant roles for each Ca(2+) pump isoform in cellular Ca(2+) regulation.

InGaP/GaAs hole barrier asymmetry determined by (0 0 2) X-ray reflections and p-type DB-RTD hole transport

InGaP/GaAs heterostructures are grown by LP-MOVPE. The InGaP layer embedded in GaAs and both the upper and the lower interface are characterized by high resolution X-ray diffraction (HR-XRD) using the (0 0 2) reflection, which is quasi forbidden for GaAs. Thin and center-symmetric InGaP–GaAs–InGaP structures embedded in GaAs result in fringes which can be described using kinematical theory. This way both a thickness and composition analysis of InGaP layers is analytically obtained. Under optimized growth conditions the GaAs-to-InGaP interface is atomically abrupt while a 1 ML InGaAs interfacial layer is determined at the InGaP-to-GaAs interface. This asymmetry results in different hole-barrier functionality which is proven by low temperature I– V-characterization of p-type double barrier resonant tunneling diode (DB-RTD).

The sodium pump in the evolution of animal cells

Plant cells and bacterial cells are surrounded by a massive cellulose wall, which constrains their high internal osmotic pressure (tens of atmospheres). Animal cells, in contrast, are in osmotic equilibrium with their environment, have no restraining surround, can take on a variety of shapes and change these from moment to moment. This osmotic balance is achieved by the action of the energy-consuming sodium pump, one of the P-type ATPase transport protein family, members of which are indeed also found in bacteria. The pump's action brings about a transmembranal electrochemical gradient of sodium ions, harnessed in a range of transport systems that couple the dissipation of this gradient to establishing a gradient of the coupled substrate. The primary role of the sodium pump as a regulator of cell volume has evolved to provide the basis for an enormous variety of physiological functions.

Novel Bacterial P-Type ATPases with Histidine-Rich Heavy-Metal-Associated Sequences

Menkes disease and Wilson disease are human disorders of copper metabolism. It has recently been shown that both are due to mutations in P-type ATPase copper transport molecules. Related heavy metal transporting ATPases have been described in several strains of bacteria. In an effort to isolate other mammalian metal transporters, we screened a human small intestine library with probes homologous to conserved sequences in the known proteins. Two novel cDNAs were isolated, which encode new members of this family. Surprisingly, they were both of bacterial origin, most likely derived from E. coli sequences transduced during library construction.

The MgATPase activity of rat cardiac sarcoplasmic reticulum is a function of the calcium ATPase protein

Magnesium-dependent ATPase (MgATPase) activity is associated with many E 1-E 2 or P-type transport ATPases including the sarcoplasmic reticulum (SR) calcium ATPase. The SR isolated from rat heart has a MgATPase activity which is 6–12 times faster than the MgATPase activity of the SR isolated from dog heart. To determine the origin of the high MgATPase activity of rat heart SR, we compared and contrasted cardiac SR isolated from both species. The preparations were similar in the following ways: (i) contamination by other organelles; (ii) the comigration of MgATPase activity with calcium-dependent ATPase (CaATPase) activity through a sucrose gradient; (iii) a similar ATPase activity sensitivity to pH and ATP concentration; (iv) the high and similar of sensitivity of ATPase activity to detergent; and (v) a similar protein profile. In both preparations, a single protein in the 105,000-Da region of polyacrylamide gels was phosphorylated by ATP, and the phosphorylated species was an acylphosphate formed in the presence and absence of calcium. Dimethyl sulfoxide, which slows acylphosphoenzyme breakdown, markedly inhibited both CaATPase and MgATPase activities of both preparations but not other enzyme activities. Importantly, the specific inhibitor of the SR calcium pump, thapsigargin, completely inhibited the CaATPase activity with an I 50 of 6–7 n m; however, a higher concentration (I 50 of 2 μ m) was required to inhibit the MgATPase activity of the rat cardiac SR. These results provide evidence that the Mg-ATPase activity of rat cardiac SR is part of the enzyme cycle of the calcium ATPase protein.

Inelastic tunneling characteristics of AlAs/GaAs heterojunctions

We report the first observation of inelastic tunneling in electronic transport perpendicular to a thin AlAs layer sandwiched between two GaAs layers. Temperature dependent I–V, first derivative (dI/dV), and second derivative (d2I/dV2) measurements were made on AlAs/GaAs double heterojunctions for a range of AlAs layer thicknesses and dopings. For p-type AlAs barriers current transport at 4.2 K was due to tunneling, and reproducible structure was seen in the second derivative spectrum. This structure was associated with the inelastic excitation of AlAs optical phonons and with a density-of-states effect caused by optical phonon-electron coupling in the GaAs. A different second derivative spectrum which also exhibited reproducible structure was obtained for n-type AlAs layers. Several possible explanations for these differences are proposed.